Tunable wavelength see-through layer stack

ABSTRACT

Aspects of the present disclosure provide a method of aligning a wafer pattern. For example, the method can include providing a wafer having a reference pattern located below a front side of the wafer, and directing a light beam to the wafer. The method can further include identifying at least one of power and a wavelength of the light beam such that the light beam is capable of passing through the wafer and reaching the reference pattern, or identifying at least one of power and a wavelength of the light beam based on at least one of a material of the wafer and a depth of the reference pattern below the front side of the wafer. The method can further include using the light beam to image the reference pattern.

INCORPORATION BY REFERENCE

This present disclosure claims the benefit of U.S. ProvisionalApplication No. 63/066,779, “Method for Producing Overlay Results withAbsolute Reference for Semiconductor Manufacturing” filed on Aug. 17,2020, which is incorporated herein by reference in its entirety.

FIELD OF THE PRESENT DISCLOSURE

The present disclosure relates generally to methods of fabricatingsemiconductor devices and specifically to overlay error.

BACKGROUND

Semiconductor fabrication involves multiple varied steps and processes.One typical fabrication process is known as photolithography (alsocalled microlithography).

Photolithography uses radiation, such as ultraviolet or visible light,to generate fine patterns in a semiconductor device design. Many typesof semiconductor devices, such as diodes, transistors, and integratedcircuits, can be constructed using semiconductor fabrication techniquesincluding photolithography, etching, film deposition, surface cleaning,metallization, and so forth.

SUMMARY

Aspects of the present disclosure provide a method of aligning a waferpattern. For example, the method can include providing a wafer having areference pattern located below a front side of the wafer, and directinga light beam to the wafer. The method can further include identifying atleast one of power and a wavelength of the light beam such that thelight beam is capable of passing through the wafer and reaching thereference pattern. The method can further include using the light beamto image the reference pattern.

In an embodiment, the wafer can further have one or more layers formedon the front side, and identifying at least one of power and awavelength of the light beam includes identifying at least one of powerand a wavelength of the light beam such that the light beam is capableof passing through the one or more layers and the wafer and reaching thereference pattern.

In an embodiment, the method can further include measuring at least oneof absorption amount and scattering amount of the light beam passingthrough the wafer to determine that the light beam is capable of passingthrough the wafer and reaching the reference pattern.

In an embodiment, the method can further include providing an infrared(IR) light source that generates the light beam. For example, the IRlight source can be an IR wavelength tunable light source. As anotherexample, the IR wavelength tunable light source can include quantumcascade lasers (QCLs). In an embodiment, the wavelength of the lightbeam can be 1-10 micrometers. For example, the second wavelength can be3.6 or 3.7 micrometer.

In an embodiment, the second pattern can be incorporated in a referenceplate located on a back side of the wafer. For example, the referenceplate can be adhered to the back side of the wafer. In anotherembodiment, the second pattern can also be projected on a surface of thewafer. In yet another embodiment, the second pattern can be formed on aback side of the wafer. In still another embodiment, the second patterncan be embedded within the wafer.

Aspects of the present disclosure further provide another method ofaligning a wafer pattern. For example, the method can also includeproviding a wafer with a reference pattern located below a front side ofthe wafer, and directing a light beam to the reference pattern. Themethod can further include identifying at least one of power and awavelength of the light beam based on at least one of a material of thewafer and a depth of the reference pattern below the front side of thewafer. The method can also include using the light beam to image thereference pattern.

In an embodiment, the wafer can further have one or more layers formedon the front side, and identifying at least one of power and awavelength of the light beam includes identifying at least one of powerand a wavelength of the light beam such that the light beam is capableof passing through the one or more layers and the wafer and reaching thereference pattern.

In an embodiment, the method can also include providing an IR lightsource that generates the light beam. For example, the IR light sourcecan be an IR wavelength tunable light source. As another example, the IRwavelength tunable light source can include quantum cascade lasers(QCLs). In an embodiment, the reference pattern is located on a backside of the wafer. In another embodiment, the reference pattern isembedded within the wafer. In yet another embodiment, the referencepattern can be incorporated in a reference plate located on a back sideof the wafer.

As can be appreciated, as fabrication progresses on a given wafer,depending on a given device being created, there can be many differentmaterials and layers. Thus each wafer at each process stage can have adifferent profile. This means a different wavelength may be needed topass through the wafer.

Of course, the order of discussion of the different steps as describedherein has been presented for clarity sake. In general, these steps canbe performed in any suitable order. Additionally, although each of thedifferent features, techniques, configurations, etc. herein may bediscussed in different places of this disclosure, it is intended thateach of the concepts can be executed independently of each other or incombination with each other. Accordingly, the present disclosure can beembodied and viewed in many different ways.

Note that this summary section does not specify every embodiment and/orincrementally novel aspect of the present disclosure or claimeddisclosure. Instead, this summary only provides a preliminary discussionof different embodiments and corresponding points of novelty overconventional techniques. For additional details and/or possibleperspectives of the present disclosure and embodiments, the reader isdirected to the Detailed Description section and corresponding figuresof the present disclosure as further discussed below.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of this disclosure that are proposed as exampleswill be described in detail with reference to the following figures,wherein like numerals reference like elements, and wherein:

FIG. 1A shows an industrial problem of overlay;

FIG. 1B shows overlay alleviation using an exemplary reference patternin accordance with some embodiments of the present disclosure;

FIG. 2 is a functional block diagram of an exemplary imaging system inaccordance with some embodiment of the present disclosure;

FIG. 3 is a graph showing absorption spectrums of some materials;

FIG. 4 is an enlarged view of a portion of coaxially aligned light beamsgenerated by the exemplary imaging system of FIG. 2;

FIG. 5A shows an enlarged top view of superimposed images of a portionof a wafer captured by the first and second image capturing devices ofthe exemplary imaging system of FIG. 2 in accordance with someembodiments of the present disclosure

FIG. 5B demonstrates exemplary image analysis for overlay calculationusing an absolute, independent reference pattern in accordance with someembodiments of the present disclosure; and

FIG. 6 is a flow chart illustrating an exemplary method in accordancewith some embodiments of the present disclosure.

DETAILED DESCRIPTION

In accordance with the present disclosure, a method of aligning a waferpattern is provided, which uses an absolute, independent referencepattern as an alignment mark for feature patterns to be aligned with,instead of being aligned with a previous pattern. The feature patternscan be formed on a front side of a wafer, and the reference pattern isindependent of the front side of the wafer. For example, the referencepattern can be formed within or below the wafer. A first light beam(e.g., an ultraviolet (UV) light beam) of a first wavelength can be usedto image the feature patterns formed on the first side of the wafer, anda second light beam (e.g., an infrared (IR) light beam) of a secondwavelength can be used to image the reference pattern formed within orbelow the wafer. In an embodiment, the second light beam can becoaxially aligned with the first light beam. As the reference pattern isformed within or below the wafer, the second light beam has to “seethrough” a portion of the thickness or the entire thickness of the waferin order to image the reference pattern. For example, the second lightbeam can have power or intensity sufficient to pass through a portion ofthe thickness or the entire thickness of the wafer, depending on whetherthe reference pattern is formed within or below the wafer, to capture animage of the reference pattern using quantum tunneling imaging, IRtransmission imaging or the like. As another example, the power and thesecond wavelength of the second light beam can be tuned, based on atleast one of a material of the wafer and a depth of the referencepattern below the front side of the wafer, for example, such that thesecond light beam is capable of passing through the wafer and reachingthe reference pattern. Therefore, UV images of the feature patterns andIR images of the reference pattern can be captured in the same lightaxis and superimposed on each other. Image analysis can then beperformed for exposure, inspection, alignment or other processing.Although the UV and IR images are captured coaxially, transmission to animage detector may or may not be coaxial. For example, the coaxiallycaptured images may be optically separated and transmitted to separateimage detectors as discussed below.

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.Further, spatially relative terms, such as “top,” “bottom,” “beneath,”“below,” “lower,” “above,” “upper” and the like, may be used herein forease of description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

The order of discussion of the different steps as described herein hasbeen presented for clarity sake. In general, these steps can beperformed in any suitable order. Additionally, although each of thedifferent features, techniques, configurations, etc. herein may bediscussed in different places of this disclosure, it is intended thateach of the concepts can be executed independently of each other or incombination with each other. Accordingly, the present disclosure can beembodied and viewed in many different ways.

Microfabrication involves forming and processing multiple films andlayers on a wafer. This can include dozens or more films stacked on awafer. Patterns applied to the wafer for various films and layers needto be aligned to previously-formed patterns. Conventionally, suchalignment is realized by using part of the wafer to form alignment marksand scribe lines. However, the present inventors recognized that thevarious film deposition, etching, and treatment techniques at timescover the alignment marks and even completely remove the alignmentmarks. With alignment marks at times covered or missing, there can beerrors applying subsequent patterns on the wafer. The term overlay oroverlay error refers to the difference between placement of givenpattern relative to a previously-placed pattern. With alignment marksroutinely destroyed, overlay error can accumulate with additionallayers, which can cause poor performance and device error.

FIG. 1A illustrates an industrial problem of overlay. Each arrow hereinhas a starting point (e.g., 111A, 111B, 121A and 131A), whichcorresponds to a position of a preceding pattern, and an endpoint orarrowhead (e.g., 111A′, 111N′, 121N′), which corresponds to a positionof a subsequent pattern. As a result, each arrow represents an overlayvalue or overlay error when the subsequent pattern is formed over orside by side with the corresponding preceding pattern. In a process110A, for example, there is no grid or reference plate when placing aninitial pattern. Thus, a starting point 111A of a first arrow is likelyto be misaligned, that is, the initial pattern can have a placementerror, for example relative to the wafer edge. Then subsequent patternstry to align based on the corresponding last pattern. As illustrated inFIB. 1A, a starting point (e.g., 111B) of a subsequent arrow overlapswith an arrowhead (e.g., 111A′) of a corresponding last or precedingarrow. In some embodiments, deterioration of alignment marks can causealignment error for subsequent patterns placed by using suchdeteriorated alignment marks. Note that even in a theoretically perfectsystem, walkout can still occur. For example, if a system patternplacement tolerance is +/−4 nm and each level references a previouslevel. Take a reference level to be 0 error. A first layer then could be+4 nm off. A second layer alignment to the first layer could be +4 nmoff, meaning the second layer is now+8 nm off the reference level. Thereare also process factors that induce or relieve stress throughoutfabrication that can induce walkout/alignment shift even with pristinealignment marks visible that can add to accumulated error.

Further, alignment marks may be destroyed at a step S120 in amanufacturing process, and placement again happens without a referencemark. Deterioration of alignment marks can cause accumulation ofalignment error with subsequent processing. Similar to the startingpoint 111A, a starting point 121A of a new arrow is likely to bemisaligned. In the example of FIG. 1A, the starting point 121A deviatesfrom an arrowhead 111N′. The process proceeds by aligning subsequentpatterns based on the corresponding last pattern until alignment marksare destroyed again at a step S130. Similarly, placement happens withouta reference mark, and a starting point 131A deviates from an arrowhead121N′. As can be seen in FIG. 1A, as layers increase, the overlay errorcan accumulate leading to poor manufacturing yield, device error, etc.Note that the process 110A is a non-limiting example. Other processes(e.g., 110B and 110C) may have different overlay values (differentarrows) and/or different steps.

FIG. 1B shows overlay alleviation using an exemplary reference patternin accordance with some embodiments of the present disclosure. Withtechniques herein, all patterns (e.g., a pattern having a starting point141A) placed on a front side (or working side) 191 of a wafer 190 arebased on a same reference pattern 102. In an embodiment, the referencepattern 102 can be located below the front side 191 of the wafer 190.For example, the reference pattern 102 can be formed on a back side 192of the wafer 190 or incorporated in the wafer 190. As another example,the reference pattern 102 can be incorporated in a reference plate (notshown in FIG. 1B), and the reference plate can be adhered to the backside 192 of the wafer 190 or incorporated in a substrate holder (notshown in FIG. 1B) that is used to hold the wafer 190. In other words,the reference pattern 102 is not affected by lithographic processes,such as etching, deposition, chemical mechanical polishing and the like,which are performed on the front side 191 of the wafer 190 in order toform patterns. Therefore, the reference pattern 102 is independent ofthe front side 191 of the wafer 190, and will be intact during thelithographic processing of the wafer 190. Accordingly, the referencepattern 102 can be used and considered absolute, or rather, independentof any patterns formed on the front side 191 of the wafer 190, and willnot be changed from various deposition and etch steps performed on thewafer 190. In an embodiment, the reference pattern 102 can be comparedto the wafer 190 when placing a new pattern. For an initial pattern,this means that the pattern can be fitted to the reference pattern 102.For subsequent patterns, this means that one or more patterns can stillbe compared to the reference pattern 102 to calculate overlay correctionto return to a same alignment.

For example, in a process 140 the reference pattern 102 can be used toalign an initial pattern on the front side 191 of the wafer 190. In oneembodiment, the reference pattern 102 can be provided in a fixedposition relative to the wafer surface such as by embedding thereference pattern 102 within the wafer 190 or providing the referencepattern 102 fixed to a back side 192 of the wafer 190. Consequently, astarting point 141A of a first arrow is aligned to the reference pattern102, whose position is demonstrated as a reference line 150. Subsequentpatterns are also aligned using the fixed absolute, independentreference pattern 102. A new photoresist layer may be formed for eachsubsequent pattern, but no alignment marks need to be formed and/ordestroyed on the wafer 190 due to the reference pattern 102. As aresult, arrows center around the reference line 150, meaning that thesubsequent patterns are aligned to the reference pattern 102. Alignmentmay occur, for example, by moving a mask of the pattern image or movingthe wafer 190 relative to the mask. Overlay error is therefore unlikelyto accumulate as more and more layers are formed.

FIG. 2 is a functional block diagram of an exemplary imaging system 200in accordance with some embodiments of the present disclosure. Forexample, the exemplary imaging system 200 can be implemented in ascanner or a stepper of a lithography system. As another example, theexemplary imaging system 200 can be implemented in a resist coatingtool, e.g., CLEAN TRACK™ ACT™12 manufactured by Tokyo Electron Ltd, theresisting coating tool containing multiple mask-specific modules such asadvance softbake oven units, edge-bead removal modules, and cleaningsystems. The exemplary imaging system 200 can coaxially align two lightbeams of different wavelengths, focus the two coaxially aligned lightbeams onto a first pattern located on a front side of a substrate (e.g.,a wafer) and a second pattern located below the first pattern,respectively, and capture images of the first and second patterns. Forexample, the exemplary imaging system 200 can include a first lightsource 210, a second light source 220, an alignment module 230, acoaxial module 240, a first image capturing device 250 and a secondimage capturing device 260. The first image capturing device 250 and thesecond image capturing device 260 can be referred to as an imagecapturing module collectively.

In an embodiment, the first light source 210 can be configured togenerate a first incident light beam of a first wavelength. For example,the first light source 210 can be a UV light source that generates afirst incident light beam of 50-400 nanometers, e.g., 266 nanometers(shown in FIG. 2 as UV_(incident)). As another example, the first lightsource 210 can be an Optowaves (Optowares Inc., Massachusetts, USA)solid state lasers, such as pumped nanosecond laser for surface imaging.

In an embodiment, the second light source 220 can be configured togenerate a second incident light beam of a second wavelength. Accordingto some aspects of the present disclosure, as an absolute, independentreference pattern shall be located below a pattern that is to be formedon a front side of a wafer and the second incident light beam is used toimage the reference pattern, the second incident light beam has to seethrough at least a portion of a thickness or even the entire thicknessof the wafer, such as a wafer 290.

For example, the second incident light beam has power or intensitysufficient to pass through the entire thickness (e.g., 750 micrometers)of the wafer 290 to capture an image of the reference pattern usingquantum tunneling imaging, IR transmission imaging or the like. Asanother example, the second light source 220 can be an IR light sourcethat generates a second incident light beam of 1-10 micrometers, e.g.,3.6 or 3.7 micrometer (shown in FIG. 2 as IR_(incident)). In anembodiment, the second light source 220 can be IR tunable quantumcascade lasers (QCLs), which can be obtained from Pranalytica, Inc.(California, USA). A QCL can consist of a periodic series of thin layersof varying material composition forming a superlattice. In a quantumcascade structure, electrons undergo intersubband transitions andphotons are thus emitted. The electrons tunnel to the next period of thequantum cascade structure and the process repeats. Therefore, QCLs canconvert electrical power into optical power and generate laser radiationin the mid wave infrared (MWIR) and long wave infrared (LWIR). Table 1below provides a concise summary of the QCL performance that has beenreported for room temperature (RT) Fabry-Pérot (FP) continuous wave (CW)operation requiring thermoelectric coolers (TEC) for thermal managementand quasi-continuous wave (QCW) operation for a laser system operation.

FP Center CW/RT Power (FP CW/RT Power (FP Wavelength Geometry) TunableQCLs Geometry) 3.8 μm >1.5 W 3.5 μm-3.9 μm >200 mW 4.0 μm >2.5 W 3.8μm-4.2 μm >300 mW 4.6 μm >4.0 W 4.3 μm-5.0 um >700 mW 5.3 μm ~500 mW 5.0μm-5.6 μm >200 mW 6.2 μm ~500 mW 5.9 μm-6.5 μm >200 mW 6.8 μm ~1 W 6.5μm-7.0 μm >200 mW 7.2 μm ~1.4 W 7.0 μm-7.5 μm >300 mW 8.2 μm ~1 W 8.0μm-8.5 μm >300 mW 9.2 μm ~2.0 W 9.2 μm-9.6 μm >500 mW 10.2 μm  ~1 W  9.6μm-10.8 μm >100 mW 10.6 μm  ~500 mW 10.3 μm-11.0 μm >100 mW 11.3 μm ~200 mW 10.9 μm-11.7 μm  >50 mW

According to evanescent wave theory, a light beam impinging at a surface(e.g., a front side 391 of the wafer 290, as shown in FIG. 4) betweentwo different media (e.g., the wafer 290 and air or liquid in immersionlithography where the coaxial module 240 is located) will have itsintensity decayed exponentially perpendicular to the surface. Thepenetration depth over which the intensity drops to 1/e (approximately37%) depends on, among other things, the wavelength of the light beam.Typical penetration depth can be a fraction of the wavelength of thelight beam, e.g., ⅕ of the wavelength, depending on the incident angleof the light beam to the surface. As the second wavelength of the secondincident light beam IR_(incident) is much longer than the firstwavelength of the first incident light beam UV_(incident), the secondincident light beam IR_(incident), with power well controlled, can becapable of passing through the entire thickness of the wafer 290.

In an embodiment, the relative position of the first (UV) light source210 and the second (IR) light source 220 can be calibrated periodically,which is also referred to as relative position of red and bluecalibration. For example, the relative position of the first lightsource 210 and the second light source 220 can be kept within a sensordynamic range which is a few decades and as such quite forgiving.Normalization, however, can be done with a stage artifact of knownrelative transmission being imaged as needed. For example, once a day sothat any relative intensity normalization can be conducted easily.Relative position or TIS tool induced shift calibrations are common tometrology stations. Relative position is recalibrated against the gridplate in real time as measurements are made. Accordingly, the exemplaryimaging system 200 can always have a real time absolute reference.Digital image capture and regression can be used.

Incoming light beam, e.g., the second light beam, will be scattered froman object, e.g., the wafer 290 and one or more layers formed thereon,which is known as Rayleigh scatter. Rayleigh scatter will influence themeasured absorption spectrum since the scattered light beam cannot reacha detector of the absorption spectrometer and will be interpreted asabsorbed light beam. The intensity of Rayleigh scatter (or absorptionspectrum or absorption amount) of the object is a function of thewavelength of the incoming light beam and varies with the material ofthe object, as shown in FIG. 3. By tuning the wavelength of the incominglight beam, a region can be found that is more transparent (i.e.,non-absorbing), thus increasing the image fidelity. Increasing the powerof the incoming light beam can realize a stronger signal at thedetector. For example, in a 10% transmission scenario if the sourcepower is 10 W, 1 W would be yielded at the detector, and a lower powersource of 1 W would only yield 100 mW at the detector. FIG. 3 also showsthat different materials, e.g., Cu, Fe, Cr etc., which may be used toform one or more layers on the wafer, have respective absorptionspectrum-wavelength functions. Therefore, the wavelength of the incominglight beam can be tuned based on the material of the one or more layersand the wafer.

In an embodiment, the alignment module 230 can be configured tocoaxially align the second incident light beam IR_(incident) with thefirst incident light beam UV_(incident). For example, the alignmentmodule 230 can include a first light beam splitter that splits the firstincident light beam UV_(incident) into two parts, one of which can betransmitted and the other of which can be reflected. In an embodiment,the first light beam splitter can be a prism. In another embodiment, thefirst light beam splitter can be a transparent plate, such as a sheet ofglass or plastic, coated on one side thereof with a partiallytransparent thin film of metal, such as aluminum, which allows one partof the first incident light beam UV_(incident) to be transmitted and theother part to be reflected. In the exemplary imaging system 200, thefirst light source 210 and the first light beam splitter can be arrangedsuch that the first incident light beam UV_(incident) is incident at a45-degree angle to the first light beam splitter.

For example, the alignment module 230 can further include a second lightbeam splitter that splits the second incident light beam IR_(incident)into two parts, one of which can be reflected and the other of which canbe transmitted. For example, the second light beam splitter can be aprism. As another example, the second light beam splitter can be a sheetof glass or plastic coated on one side thereof with a thin film ofaluminum, which allows one part of the second incident light beamIR_(incident) to be reflected and the other part to be transmitted. Inthe exemplary imaging system 200, the second light source 220 and thesecond light beam splitter can be arranged such that the second incidentlight beam IR_(incident) is incident at a 45-degree angle to the secondlight beam splitter.

For example, the alignment module 230 can further include a third beamsplitter that allows light beams of different wavelengths to be eitherreflected or transmitted. For example, the third beam splitter can be atransparent plate coated on one side thereof with a dichroic materialthat allows the first incident light beam UV_(incident) of the firstwavelength that is transmitted from the first light beam splitter to bereflected, and the second incident light beam IR_(incident) of thesecond wavelength that is transmitted from the second light beamsplitter to be transmitted. In an embodiment, the third beam splitter isdesigned and located such that the transmitted second incident lightbeam IR_(incident) is coaxially aligned with the reflected firstincident light beam UV_(incident) and the transmitted second incidentlight beam IR_(incident) and the reflected first incident light beamUV_(incident) can travel to the wafer 290 along the same light path.

In an embodiment, the coaxial module 240 can be configured to focus thefirst incident light beam UV_(incident) reflected from the third beamsplitter onto a first pattern 301 (shown in FIG. 4) located on a frontside 391 of the wafer 290, and focus the second incident light beamIR_(incident) transmitted from the third beam splitter onto a secondpattern 302 (or a reference pattern) located below the first pattern301. For example, the coaxial module 240 can be designed and configuredto adjust the tolerances of the placement (i.e., depth of focus (DOF))of the first pattern 301 and the second pattern 302. For example, alevel sensor can be used to track the top of the first pattern 301 andsubtract the height of the first pattern 301 by the height of the wafer290 to auto-adjust the DOFs of the coaxially aligned first incidentlight beam UV_(incident) and second incident light beam IR_(incident)simultaneously. With deep UV (DUV) light, photoresist damage can benegligible. A 250-micrometer field of view (FOV) herein corresponds withabout 60 nanometers per pixel in the case of 4K resolution. It issufficient for resolution of 0.1-nanometer registration errormeasurement. Having sufficient power or intensity of light source canmitigate any shadowing of metal layers. While FIG. 4 shows imaging of aphysical pattern formed in the wafer 290, images of a pattern to beformed (i.e., prior to exposure to activating light) may be realized bylight having a wavelength that does not activate photoresist in thewafer, for example.

In an embodiment, the coaxial module 240 can include 2-12 individualoptical elements, e.g., 6 optical elements. Each of the optical elementscan include sapphire, AN, MgF, CaF, BaF, LiF, Ge, Si, etc.

The first incident light beam UV_(incident) can be reflected by thefirst pattern 301 to form a first reflection light beam UV_(reflection).The first reflection light beam UV_(reflection) can be reflected by thethird beam splitter and the first light beam splitter sequentially andcaptured by the first image capturing device 250, and the first imagecapturing device 250 can form a corresponding first image of the firstpattern 301. For example, the first image capturing device 250 can beDataRay camera. The second incident light beam IR_(incident) can bereflected by the second pattern 302 to form a second reflection lightbeam IR_(reflection). The second reflection light beam IR_(reflection)can be transmitted by the third beam splitter and the second light beamsplitter sequentially and captured by the second image capturing device260, and the second image capturing device 260 can form a correspondingsecond image of the second pattern 302. For example, the second imagecapturing device 260 can be a high speed, high definition middlewavelength IR (MWIR) camera, e.g., FLIR X8500 MWIR. In an embodiment,image analysis can be performed on the first image and the second imageto calculate an overlay value to determine the placement of the firstpattern 301. For example, the image analysis can be accomplished bysuperimposing the first image of the first pattern 301 and the secondimage of the second pattern 302 on each other, and identifyingcoordinate locations of the first pattern 301 relative to the secondpattern 302. In some embodiments, the image analysis can be performed inreal time so that the placement of the first pattern 301 can be adjustedin real time.

In an embodiment, the alignment module 230 can further include a firstlens set and a second lens set. For example, the first lens set caninclude reflective and/or refractive optics that collimate the firstincident light beam UV_(incident) generated by the first light source210 and direct the collimated first incident light beam UV_(incident) tothe first light beam splitter. As another example, the second lens setcan also include reflective and/or refractive optics that collimate thesecond incident light beam IR_(incident) generated by the second lightsource 220 and direct the collimated second incident light beamIR_(incident) to the second light beam splitter.

In an embodiment, the exemplary imaging system 200 can further include athird lens set 270 and a fourth lens set 280. For example, the thirdlens set 270 can include reflective and/or refractive optics that focusthe first reflection light beam UV_(reflection) onto the first imagecapturing device 250. As another example, the fourth lens set 280 canalso include reflective and/or refractive optics that focus the secondreflection light beam IR_(reflection) onto the second image capturingdevice 260.

In an embodiment, the exemplary imaging system 200 can further includeoptics that can capture diffracted light beams outside of the coaxialmodule 240 and direct them to the first image capturing device 250 andthe second image capturing device 260.

In the exemplary embodiment shown in FIG. 4, the first pattern 301 canbe included in a photomask (not shown) that is located on the front side391 of the wafer 290. In an embodiment, the photomask can be placed indirect contact with the wafer 290 in a contact printing system. Inanother embodiment, the photomask can be placed away from the wafer 290in a proximity printing system or in a projection printing system.

In the exemplary embodiment shown in FIG. 4, the second pattern 302 islocated on a back side 392 of the wafer 290, and the second incidentlight beam IR_(incident) has power sufficient to pass through the entirethickness of the wafer 290 to capture the second image of the secondpattern 302 using quantum tunneling imaging, IR transmission imaging orthe like. In an embodiment, the second pattern 302 can be formed on areference plate 310. For example, the reference plate 310 can be a gridplate with 20 micrometers by 20 micrometers squares, nearly perfectlyaligned, and the second pattern 302 can be a corner point of at leastone of the squares. As another example, the reference plate 310 caninclude at least one of a point, a line, a corner, a box, a number, amark, or any other pattern that is suitable for alignment purpose, andthe second pattern 302 can be one of these. In an embodiment, thereference plate 310 can be adhered to the back side 392 of the wafer290. Accordingly, the reference plate 310 and the wafer 290 can functionas one module. In another embodiment, the reference plate 310 can beincorporated in a substrate holder 320 of a photolithography scanner orstepper. Although each time a given wafer may be placed on the substrateholder 320 in a different position or orientation as compared to aprevious placement, this does not matter. For a given new pattern to beplaced or exposed, the wafer can be imaged with the reference plate 310,e.g., the grid plate. The reference plate 310 can then provide arelatively reference point for identifying vectors to two or morepoints, from which vector analysis can be used to calculate an overlaycorrection adjustment in a next exposure. For example, when the wafer290, if having no pattern yet, is placed over the reference plate 310,the wafer 290 will be coarsely pre-aligned to the reference plate 310.As another example, when the wafer 290, if having an existing patternalready, is placed over the reference plate 310, the existing patternand the reference plate 310 can be co-axially aligned. In a conventionallithography process, measurement errors caused by wafer back sidescratches, back side dust and/or substrate distortion due to heat, mayimpact overlay, but conventional overlay systems are often blind tothese problems. Techniques herein include an independent reference plateand high spatial resolution to overcome these problems.

In an embodiment, the second pattern 302 can be formed on the back side392 of the wafer 290, and the second incident light beam IR_(incident)also has power sufficient to pass through the entire thickness of thewafer 290 to capture the second image of the second pattern 302 usingquantum tunneling imaging, IR transmission imaging or the like. Othertechniques can include embedding the second pattern 302 (e.g., gridlines) in the wafer 290 such as using a radioactive or fluorescentmaterial.

In an embodiment, the second pattern 302 can be formed on the front side291 of the wafer 290, and then a layer of silicon and/or silicon oxideis deposited thereon. For example, the layer of silicon and/or siliconoxide can have a thickness of 1-5 micrometers so that the second pattern302 is effectively “embedded” in the wafer 290 and patterns can beformed on the layer of silicon and/or silicon oxide. Accordingly, thesecond incident light beam IR_(incident) has to have power sufficient topass through the layer of silicon and/or silicon oxide in order tocapture the second image of the second pattern 302 using quantumtunneling imaging, IR transmission imaging or the like. As anotherexample, the second pattern 302 can be formed on the back side 292 ofthe wafer 290 before a protection layer, such as silicon or siliconoxide formed on the back side 292 of the wafer 290. Consequently, thesecond pattern 302 can also be embedded in the wafer 290. Accordingly,the second incident light beam IR_(incident) has to have powersufficient to pass through the entire thickness of the wafer 290 inorder to capture the second image of the second pattern 302 usingquantum tunneling imaging, IR transmission imaging or the like. In anembodiment, the second pattern 302 can be formed on a front side of acarrier wafer before the front side of the carrier wafer is bonded to aback side of a target wafer (e.g., the back side 392 of the wafer 290).As a result, the second pattern 302 can be sandwiched between thecarrier wafer and the target wafer, which together function as onewafer. Accordingly, the second incident light beam IR_(incident) has tohave power sufficient to pass through the entire thickness of the targetwafer in order to capture the second image of the second pattern 302using quantum tunneling imaging, IR transmission imaging or the like. Insome embodiments, light projection can also be used. For example, thesecond pattern 302 can be a projected grid that does not physicallyexist in the wafer 290, on a substrate holder or as a grid plate underthe substrate holder. In some embodiments, the second pattern 302 may bea combination of physical marks and light projection. For example,physical reference marks may be provided on a peripheral region of asubstrate holder that is not covered by a wafer placed on the substrateholder, and light projections can complete the reference pattern in thearea of the wafer such that tunneling may not be necessary.

FIG. 5A shows an enlarged top view of superimposed images of a portionof the wafer 290 captured by the first image capturing device 250 andthe second image capturing device, the portion including the firstpattern 301 and the second pattern 302, in accordance with someembodiments of the present disclosure. FIG. 5B demonstrates exemplaryimage analysis for overlay calculation using the first pattern 301,which acts as a reference pattern in an alignment process, in accordancewith some embodiments of the present disclosure. FIGS. 5A and 5B showhow the absolute, independent first pattern 301 can be used to calculatean overlay value of two patterns. This can be done by knowing eachcommon reference pattern to a co-ordinate system and using thatreference pattern to know “where” each pattern is in that co-ordinatesystem. Once that is known, for example the distance between each layer,the vector calculation required to extract the overlay value is donewith simple vector algebra. From that point, it is basic co-ordinategeometry. One can think of it as mix-match overlay (MMO) with a goldentool always there for oneself under the stage.

In an embodiment, the first pattern 301 (denoted by a point M), e.g.,the corner of one of the squares of the grid plate with 20 micrometersby 20 micrometers squares, can be considered absolute orwafer-independent and used to calculate an overlay value between thesecond pattern 302 (denoted by a point N) and a third pattern 401(denoted by a point P) that is formed subsequent to the formation of thesecond pattern 302. By superimposing the second pattern 302 on the firstpattern 301, a coordinate difference or vector {right arrow over (MN)}from the point M of the first pattern 301 to the point N of the secondpattern 302 can be determined. Likewise, by superimposing the thirdpattern 401 on the first pattern 301, another coordinate difference orvector {right arrow over (MP)} from the point M of the first pattern 301to the point P of the third pattern 401 can also be determined. Then, anoverlay value {right arrow over (NP)} between the point N and the pointP can be calculated: {right arrow over (NP)}={right arrow over(MP)}−{right arrow over (MN)}.

Further, with coordinate locations of points (e.g., N(Wx, Wy)) from thesecond pattern 302 and coordinate locations of points (e.g., P(Bx, By))from the third pattern 401, an overlay value or shift from the secondpattern 302 to the third pattern 401 can be determined. This overlayvalue can then be used to place the third or subsequent pattern tocorrect overlay relative to the independent reference pattern, e.g., thefirst pattern 301. In some embodiments, having a reference image that isuniform for every image comparison enables correcting adjacent patternsas well as keeping overlay corrections based on an initial line orabsolute reference. Regarding concerns about critical dimension (CD)variation effects for resist layers, techniques herein can extractcoordinates of the patterns without pattern CD variation effects for aresist layer and an under-layer thereof (e.g., metal resist patternscover most of via patterns). CD variation effects for resist layers canbe an issue for alignment and be ignored by overlay measurement teams asnegligible. Techniques herein are far improved as the reference patternitself is a far better indication of pattern placement than an alignmentmark that suffers from CD's astigmatism and Zernike induced offset frompatterns. Note that in some embodiments, superimposing images is notnecessary. Coordinate location data can be collected from the referenceplate and the working surface of the wafer, and then vector analysis canbe used to determine a gross offset or an overlay value.

FIG. 6 is a flow chart illustrating an exemplary method 600 for aligninga wafer pattern (e.g., the second pattern or reference pattern 302) inaccordance with some embodiments of the present disclosure. Theexemplary method 600 can be applied to the exemplary imaging system 200.In various embodiments, some of the steps of the exemplary method 600shown can be performed concurrently or in a different order than shown,can be substituted by other method steps, or can be omitted. Additionalmethod steps can also be performed as desired.

At step S610, a wafer with a reference pattern located below a frontside of the wafer can be provided. For example, the wafer 290 having thesecond pattern 302 located below the front side 391 can be provided. Inan embodiment, one or more layers can also be formed on the front sideof the wafer. In the exemplary embodiment shown in FIG. 4, the secondpattern 302 is located on the back side 392 of the wafer 290. In anembodiment, the second pattern 302 can be formed on the reference plate310, such as a grid plate with 20 micrometers by 20 micrometers squares,nearly perfectly aligned, and the second pattern 302 can be a cornerpoint of at least one of the squares. For example, the reference plate310 can be placed on or adhered to the back side 392 of the wafer 290.As another example, the reference plate 310 can be incorporated in asubstrate holder 320 of a photolithography scanner or stepper. In anembodiment, the second pattern 302 can be formed on the back side 392 ofthe wafer 290. Other techniques can include embedding the second pattern302 (e.g., grid lines) in the wafer 290 such as using a radioactive orfluorescent material. In an embodiment, the second pattern 302 can beformed on the front side 291 of the wafer 290, and then a layer ofsilicon and/or silicon oxide is deposited thereon. For example, thelayer of silicon and/or silicon oxide can have a thickness of 1-5micrometers so that the second pattern 302 is effectively “embedded” inthe wafer 290 and patterns can be formed on the layer of silicon and/orsilicon oxide. As another example, the second pattern 302 can be formedon the back side 292 of the wafer 290 before a protection layer, such assilicon or silicon oxide formed on the back side 292 of the wafer 290.Consequently, the second pattern 302 can also be embedded in the wafer290. In an embodiment, the second pattern 302 can be formed on a frontside of a carrier wafer before the front side of the carrier wafer isbonded to a back side of a target wafer (e.g., the back side 392 of thewafer 290). As a result, the second pattern 302 can be sandwichedbetween the carrier wafer and the target wafer, which together functionas one wafer.

At step S620, a light source can be provided to generate a light beam.In an embodiment, the light source can be an IR light source, e.g., thesecond light source 220, and the light beam can be an IR light beam,e.g., the second incident light beam IR_(incident). For example, the IRlight source can be an IR wavelength tunable light source, such as QCLs.As another example, the wavelength of the light beam can be 1-10micrometers, such as 3.6 or 3.7 micrometer.

At step S630, the light beam can be directed to the reference pattern.For example, the light beam can be guided and focused by the alignmentmodule 230 and the coaxial module 240 of the exemplary imaging system200 to the second pattern 302.

At step S640, at least one of power and a wavelength of the light beamcan be tuned and identified. In an embodiment, the power and thewavelength of the light beam can be tuned such that the light beam iscapable of passing through the wafer and reaching the reference patternin an embodiment, or passing through the one or more layers and thewafer and reaching the reference pattern in another embodiment. Forexample, the power and wavelength of the light beam generated by QCLscan be tuned based on at least one of an absorption spectrum (orabsorption amount) and a scattering spectrum (or scattering amount) ofthe light beam obtained by a detector by, for example, stepwise movingthrough different wavelengths to identify absorption and scattering dataat different stages of fabrication. “Dark areas” or the wavelengths thatpass through the one or more layers and the wafer with an acceptablelevel of absorption and scattering, e.g., below a predeterminedthreshold, can thus be found. In another embodiment, the power and thewavelength of the light beam can be tuned and identified based on atleast one of a material of the wafer and a depth of the referencepattern below the front side of the wafer. For example, a firstwavelength of the light beam can be used for passing through the waferduring front-end-of-line processing, and a second wavelength of thelight beam can be used for passing through the one or more layers andthe wafer after the layers (e.g., Cu, Fe, Cr etc. layers) have beenadded as the wafer and the layers may have different absorptionspectrum-wavelength functions, as shown in FIG. 3.

At step S650, the light beam can be used to image the reference pattern.For example, the light beam with the tuned and identified power andwavelength can be used to image the reference pattern.

A reference pattern used for patterning herein can be considered asabsolute in one way, and relative in another. For example, the referencepattern may keep or maintain fixed grid lines (or points or corners orboxes or any other suitable shapes) and is not changed from variousdeposition and etch steps on the wafer. In an embodiment, this can be agrid plate integrated with a stage or substrate holder. In this way thegrid plate is absolute because the same physical grid plate is usedthroughout processing of the wafer, but relative because the physicalgrid plate is not fixed to the wafer itself and may be moved relative tothe wafer throughout wafer processing. Although each time a given waferis placed on the stage, it may be in a different location or orientationas compared to a previous placement; this does not matter. For a givennew pattern to be placed or exposed, the wafer is imaged with thereference grid. The reference grid can then provide a relative referencepoint for identifying vectors to two or more points, from which vectoranalysis can be used to calculate an overlay correction adjustment in anext exposure.

The exemplary imaging system 200 and the exemplary method 600 can beimplemented as standalone coaxial metrology system and method that canoperate in combination with a lithography tool, integrated track coaxialmetrology system and method with feed forward to linked lithographycells, or active coaxial metrology system and method that can beembedded in a lithography tool for real time correction.

Aspects of the present disclosure provide an imaging method, which canprovide an accurate and precise alignment mechanism that does not relyon conventional alignment marks formed on a front surface of a wafer.Instead, with reference to a pattern or grid within/below the wafer, areliable reference pattern can be repeatedly accessed for precise andaccurate registration and alignment of subsequent patterns. Thetechniques herein will wipe out the need for traditional overlay marks.These novel paradigms for overlay can require no clear outs, no loss inreal-estate and no complex scribe line design, making silicon areautilization improved and no complex integrations for alignment marks.The exemplary reference pattern disclosed herein will not be impactedand wiped out by unfavorable processes that are making devices insteadof the alignment marks, as they often are conventionally. Overlayplacement accuracy can also now be measured from the very first layerwhere the second pattern is located, as the reference pattern is now notonly near perfect every time but hidden right under the stage therealways.

In the preceding description, specific details have been set forth, suchas a particular geometry of a processing system and descriptions ofvarious components and processes used therein. It should be understood,however, that techniques herein may be practiced in other embodimentsthat depart from these specific details, and that such details are forpurposes of explanation and not limitation. Embodiments disclosed hereinhave been described with reference to the accompanying drawings.Similarly, for purposes of explanation, specific numbers, materials, andconfigurations have been set forth in order to provide a thoroughunderstanding. Nevertheless, embodiments may be practiced without suchspecific details. Components having substantially the same functionalconstructions are denoted by like reference characters, and thus anyredundant descriptions may be omitted.

Various techniques have been described as multiple discrete operationsto assist in understanding the various embodiments. The order ofdescription should not be construed as to imply that these operationsare necessarily order dependent. Indeed, these operations need not beperformed in the order of presentation. Operations described may beperformed in a different order than the described embodiment. Variousadditional operations may be performed and/or described operations maybe omitted in additional embodiments.

“Substrate” or “target substrate” as used herein generically refers toan object being processed in accordance with the present disclosure. Thesubstrate may include any material portion or structure of a device,particularly a semiconductor or other electronics device, and may, forexample, be a base substrate structure, such as a semiconductor wafer,reticle, or a layer on or overlying a base substrate structure such as athin film. Thus, substrate is not limited to any particular basestructure, underlying layer or overlying layer, patterned orun-patterned, but rather, is contemplated to include any such layer orbase structure, and any combination of layers and/or base structures.The description may reference particular types of substrates, but thisis for illustrative purposes only.

Those skilled in the art will also understand that there can be manyvariations made to the operations of the techniques explained abovewhile still achieving the same objectives of the present disclosure.Such variations are intended to be covered by the scope of thisdisclosure. As such, the foregoing descriptions of embodiments of thepresent disclosure are not intended to be limiting. Rather, anylimitations to embodiments of the present disclosure are presented inthe following claims.

What is claimed is:
 1. A method, comprising: providing a wafer having areference pattern located below a front side of the wafer; directing alight beam to the wafer; identifying at least one of power and awavelength of the light beam such that the light beam is capable ofpassing through the wafer and reaching the reference pattern; and usingthe light beam to image the reference pattern, wherein the referencepattern is incorporated in a reference plate located below the wafer. 2.The method of claim 1, wherein the wafer further has one or more layersformed on the front side, and identifying at least one of power and awavelength of the light beam includes identifying at least one of powerand a wavelength of the light beam such that the light beam is capableof passing through the one or more layers and the wafer and reaching thereference pattern.
 3. The method of claim 1, further comprisingproviding an infrared (IR) light source, the light source generating thelight beam.
 4. The method of claim 3, wherein the IR light source is anIR wavelength tunable light source.
 5. The method of claim 4, whereinthe IR wavelength tunable light source includes quantum cascade lasers(QCLs).
 6. The method of claim 3, wherein the wavelength of the lightbeam is 1-10 micrometers.
 7. The method of claim 3, wherein thereference pattern is imaged via IR transmission imaging.
 8. The methodof claim 1, further comprising: measuring at least one of absorptionamount and scattering amount of the light beam passing through the waferto determine that the light beam is capable of passing through the waferand reaching the reference pattern.
 9. A method, comprising: providing awafer having a reference pattern located below a front side of thewafer; directing a light beam to the wafer; identifying at least one ofpower and a wavelength of the light beam based on at least one of amaterial of the water and a depth of the reference pattern below thefront side; and using the light beam to image the reference pattern,wherein the reference pattern is incorporated in a reference platelocated below the wafer.
 10. The method of claim 9, wherein the waferfurther has one or more layers formed on the front side, and identifyingat least one of power and a wavelength of the light beam includesidentifying at least one of power and a wavelength of the light beamsuch that the light beam is capable of passing through the one or morelayers and the water and reaching the reference pattern.
 11. The methodof claim 9, further comprising providing an IR light source, the IRlight source generating the light beam.
 12. The method of claim 11,wherein the IIS light source is an IR wavelength tunable light source.13. The method of claim 12, wherein the IR wavelength tunable lightsource includes quantum cascade lasers (QCLs).
 14. The method of claim9, wherein the reference pattern is imaged via IR transmission imaging.15. A method, comprising: providing a wafer having a reference patternlocated below a front side of the wafer; directing a light beam to thewafer; identifying at least one of power and a wavelength of the lightbeam such that the light beam is capable of passing through the waferand reaching the reference pattern; and using the light beam to imagethe reference pattern, wherein the reference pattern is projected on aback side of the wafer.
 16. The method of claim 15, wherein e referencepattern does not physically exist in the wafer.